Locating techniques are known that are based on three different classes of techniques:                so-called time of arrival (ToA) or differential time of arrival (DToA) techniques. DToA techniques make use of a measurement of propagation times δτi between a reference terminal and the terminal that is to be located, also referred to as the obstacle terminal, and a corresponding distance cδτi where c is the propagation speed of the wave. These techniques require a plurality of mutually synchronized reference terminals in order to locate the obstacle terminal. DToA techniques measure relative time differences between the terminals compared with a reference terminal common to the other reference terminals. It is necessary for the time reference to be common. Thus, the relative position of obstacles is determined by estimating the relative propagation times of waves from the obstacle terminal to the reference terminals and this requires at least three different reference terminals that are mutually synchronized in order to find a two-dimensional position corresponding to the point of intersection of the locuses of the points associated with each propagation time;        other techniques that are much less accurate rely on the network identification of the cell in which the obstacle terminal is to be found and the identification of the obstacle terminal in that cell, known as its cell identification (CID), in order to locate the terminal. Locating the terminal within the cell makes use of a so-called timing advance (TA) parameter and constitutes a very approximate estimate of position, associated with the size of the cell. One such technique is described for example in the document: 3GPP TS 25.305. (March 2002). “Stage 2 functional specification of UE positioning in UTRAN”; and        still other techniques, known as angle of arrival (AoA) techniques, take account of estimated angles for waves being received at each reference terminal, whenever the obstacle terminal transmits a signal. Thus, the relative positions of the obstacles are determined by the point of intersection between the locuses of points described by the arrival directions of the waves at the obstacle terminal, as proposed in the document by N. Delligiannis, S. Louvros, S. Kotsopoulos, “Optimizing location positioning using hybrid ToA-AoA techniques in mobile cellular networks”, Mobimedia '07 Conference, May 2007. These techniques require an intelligent antenna at each reference terminal. Nevertheless, they present position inaccuracy that results from the multi-path nature of the propagating waves, giving rise to distinct arrival directions associated with various obstacles (diffusers) that affect the signal being received. A cost function is proposed in order to improve the estimate, taking account of the power received at each reference terminal and the angular dispersion.        
There also exist techniques that hybridize the AoA and ToA techniques in order to improve the locating of terminals, as proposed in the document by N. Delligiannis, S. Louvros, “Hybrid TOA-AOA location positioning techniques in GSM networks”, Wireless Personal Communication, May 2009, Springer. Under all circumstances, those techniques require fine synchronization between a plurality of reference terminals (ToA technique) or indeed an intelligent antenna (AoA technique).
The principle of two-dimensional ToA and DToA techniques consists in measuring the time interval δτi between the signal transmitted by the obstacle terminal and the signal received by the reference terminals. This measurement makes it possible to calculate the relative distances between the terminals using a differential technique. Each interval δτi that corresponds to the propagation delay of the signal between a reference terminal Ti and the obstacle terminal A, is associated with a radius ri=cδτi of a circle Ci of center Ti, where Ti is the position of the reference terminal. Using a simple approach in a two-dimensional plane, the obstacle terminal A is located by considering the point of intersection of three circles of radius ri=cδτi, as shown in FIG. 1a, where c is the propagation speed of the wave in air. The exact position of the obstacle terminal, considering the point of intersection of the three circles, is valid only if the third circle coincides exactly with one of the points of intersection of the other two circles.
More generally, and as shown in FIG. 1b, when a connection might be obstructed, the point of intersection of the three circles leads to an overlap zone and not to a point. This zone is defined by three points corresponding to the point of intersection of the circles taken in pairs and situated inside the third circle. The barycenter (or center of gravity) of the three points of intersection gives an approximation to the position of the terminal, as described in the document by N. Delligiannis, S. Louvros, S. Kotsopoulos, “Optimizing location positioning using hybrid ToA-AoA techniques in mobile cellular networks”, Mobimedia '07 Conference, May 2007.
Using a more complete approach that results from processing the problem in three dimensions and making use of a DToA technique that takes account of the relative positions of the reference terminals (Ti) compared to one of the reference terminals (T1) taken as a reference, it is shown that the position of the obstacle terminal A then results from the point of intersection of three hyperbolas or three ellipses, thereby requiring four synchronized signals to be received at four reference terminals Ti. By way of example, that is demonstrated for locating boats by using beacons in the article by B. T. Fang, “Simple solutions for hyperbolic and related position fixes”, IEEE Transactions on Aerospace and Electronic Systems, 26(5), pp. 748-753, 1990.
A drawback of those ToA and DToA techniques of the prior art lies in the need to use at least three reference terminals that are mutually synchronized in accurate manner. Furthermore, those techniques present an error associated with the synchronization between the reference terminals and with the quality of the point-to-point connection (visibility or obstructed link giving rise to an indirect path between the obstacle terminal and the reference terminal). The obstructed nature of the link increases the propagation time without changing the distance between the two terminals. This modification to the propagation time takes account of the indirect path of the wave between the two terminals. This leads to an error on the estimated distance between the two terminals. Proposals have been made for enhanced ToA (E-ToA) techniques that make use of cost functions. Those techniques consist in weighting the error introduced for each point-to-point link (the technique described in particular in the document by G. Turin, W. Jewell, T. Johnston, “Simulation of urban vehicle-monitoring systems”, IEEE Trans. Vehic. Tech., Vol. VT-21, February 1972, pp. 9-16) and in estimating position by a least squares method. An improvement consisting in refining position in an iterative method is also proposed in the document by N. Delligiannis, S. Louvros, S. Kotsopoulos, “Optimizing location positioning using hybrid ToA-AoA techniques in mobile cellular networks”, Mobimedia '07 Conference, May 2007. In contrast, those E-ToA techniques continue to require synchronization between a plurality of reference terminals.
There therefore exists a need for a novel technique of locating terminals that does not require a plurality of reference terminals (at least three) to be synchronized, while nevertheless giving a location that is reliable and accurate, with implementation complexity that is small.